The invention relates to a novel process for preparing conductive polymers in the presence of polyanions, to aqueous or nonaqueous dispersions or solutions prepared by this process and to their use.
Conductive polymers are gaining increasing economic significance since polymers have advantages over metals with regard to processability, to weight and to the controlled adjustment of properties by chemical modification. Examples of known π-conjugated polymers are polypyrroles, polythiophenes, polyanilines, polyacetylenes, polyphenylenes and poly(p-phenylene-vinylenes). Layers of conductive polymers have various industrial uses, for example as a polymeric counterelectrode in capacitors, as an antistatic coating or for through-contacting of electronic circuit boards.
Conductive polymers are prepared by chemical or electrochemical, oxidative means from monomeric precursors, for example optionally substituted thiophenes, pyrroles and anilines and their respective derivatives which may be oligomeric. Especially chemically oxidative polymerization is widespread, since it is technically simple to achieve in a liquid medium and on various substrates.
A particularly important and industrially utilized polythiophene is poly(ethylene-3,4-dioxythiophene) (PEDOT or PEDT), which is prepared by chemically polymerizing ethylene-3,4-dioxythiophene (EDOT or EDT) and which, in its oxidized form, has very high conductivities and is described, for example, in EP 339 340 A2. An overview of numerous poly(alkylene-3,4-dioxythiophene) derivatives, especially poly(ethylene-3,4-dioxythiophene) derivatives, and the monomer units, syntheses and applications thereof is given by L. Groenendaal, F. Jonas, D. Freitag, H. Pielartzik & J. R. Reynolds, Adv. Mater. 12, (2000) p. 481-494.
Particular industrial significance has been gained by dispersions of PEDOT with polystyrenesulphonic acid (PSS), as disclosed, for example, in EP 0440 957. From these dispersions, it is possible to obtain transparent, conductive films which have found a multitude of applications. However, the conductivity of the layers obtained from these dispersions and the transmission of these layers is still too low in order, for example, to replace indium tin oxide (ITO) in touchscreens or organic light-emitting diodes. Layers of indium tin oxide (ITO) feature, for example, a conductivity of more than 5000 S/cm and, with a transmission of 90%, surface resistances between 5 and 20 ohms per square (ohm/sq) are achieved.
J. Y. Kim et al. showed that the use of polar high boilers can significantly enhance the conductivity of a PEDT/PSS film (J. Y. Kim et al., Synthetic Metals, 126, 2002, p. 311-316). The addition of dimethyl sulphoxide (DMSO) to a PEDT/PSS dispersion enhanced the conductivity by two orders of magnitude, specifically from 0.8 S/cm to 80 S/cm. The use of DMSO leads to transparent conductive films without haze, such that DMSO is very suitable as an additive for increasing conductivity. However, a conductivity of 80 S/cm is still insufficient in order, for example, to replace indium tin oxide (ITO) in touchscreens or organic light-emitting diodes.
JP 2006328276 enhances the conductivity of a PEDT/PSS dispersion through the use of succinimide, through which conductivities of 200-1000 S/cm can be achieved. However, succinimide is only of limited suitability for producing transparent, conductive layers, since it features a melting point of 123-135° C. and a boiling point of 285-290° C. Under customary drying conditions of 100-200° C., succinimide, unlike dimethyl sulphoxide, therefore remains in the final conductive film and forms crystalline regions there, which leads to haziness of the film. The high proportion of succinimide therefore leads a film. This procedure is therefore also unsuitable for obtaining transparent, high-conductivity layers.
JP 2001323137 discloses a PEDT synthesis in the presence of polyanions which is carried out under a nitrogen atmosphere.
EP 1323764 and WO 03048227 state that the polymerization of EDT in the presence of polyanions in a low-oxygen medium leads to an improved conductivity of the reaction product. In this process, it is important that the oxygen concentration at the time of addition of an initiator is less than 3 mg/l of reaction medium. The polymerization is then performed under atmospheric pressure. This process can produce conductive films which reach surface resistances of 2900 ohm/sq to 1200 ohm/sq. These surface resistances are likewise still insufficient in order to replace, for example, ITO in touchscreens or organic light-emitting diodes.
There was therefore still a need for processes for preparing conductive polymers which have a high electrical conductivity and a high transparency in the visible region, and which can be processed readily.